The invention relates to a method for the operation of a power output stage, in particular of an electrical machine, which comprises a bridge circuit having at least one half-bridge, wherein the half-bridge has two series-connected power semiconductor switches which, in a normal operating mode, are supplied with an operating control voltage in order to set a power voltage, and wherein the power voltage of the respective power semiconductor switch is detected for the purpose of short-circuit monitoring.
The invention also relates to an apparatus for the operation of a power output stage, in particular of an electrical machine, in particular for performing the abovementioned method, having a bridge circuit which comprises at least one half-bridge, wherein the half-bridge has two series-connected power semiconductor switches which, in a normal operating mode, are supplied with an operating control voltage in order to set a power voltage, and having a monitoring device which detects the power voltage of the respective power semiconductor switch for the purpose of short-circuit monitoring.
Methods and apparatuses of the type mentioned at the outset are known from the prior art. In drive apparatuses for motor vehicles comprising an electric drive, for example hybrid drive apparatuses or drive apparatuses for vehicles driven only electrically, power output stages are usually provided for controlling the respective electrical machine, said power output stages comprising power semiconductor switches such as MOSFETs (metal-oxide semiconductor field-effect transistors) or IGBTs (insulated-gate bipolar transistors) and correspondingly associated freewheeling diodes. In order to control polyphase, permanently and/or separately excited electrical machines, one half-bridge of the power output stage is usually assigned to each phase. The respective half-bridge comprises two series-connected power semiconductor switches which, in a normal operating mode, are supplied with an operating control voltage in order to set a desired power voltage.
In order to ensure operating safety, it is also known to monitor the power voltage of the respective power semiconductor switch or at or across the respective power semiconductor switch in order to be able to detect a short-circuit situation. If one of the power semiconductor switches goes into short circuit owing to a failure, then the logic of the half-bridge can no longer maintain the nominal pulse pattern predefined for the electrical machine for the output of torque; in particular, if the second series-connected power semiconductor switch switches despite the first power semiconductor switch being short-circuited. In this situation, a current flows which is generally impermissibly high and, in the worst case, leads to fires in the power circuit since the electrical energy which is usually switched by the power semiconductor switches is very high and the voltage range is so large that arcs possibly remain for a long time. Preferably, the driver circuit of the two power semiconductor switches monitors the respective power voltage. If said power voltage is too high in the switched-on state, it is assumed that an overcurrent is flowing and the entire semiconductor bridge is switched into freewheeling mode by switching-off or deactivating the power semiconductor switches. This is also known as desaturation monitoring or Dsat monitoring. In this case, use is made of the property of the power semiconductor switches whereby they cause a disproportionate increase in the power voltage when the current is too large, the so-called desaturation current. The control voltage provided for the switches is normally high enough that voltages which are as small as possible are dropped in the event of operating currents in the nominal range of the half-bridge or the inverter, that is to say considerably below the desaturation current, as a result of which switching and on-state losses of the semiconductors are reduced or minimized.
For the safety of the drive apparatus, it is now essential that said apparatus should only go into operation when it is ensured that no uncontrolled torque changes which could make the vehicle go out of control act on the drivetrain. Therefore, safety-related hidden faults must be ruled out or reduced to a tolerable residual risk through testing or redundancies. In the case of the desaturation monitoring, this is problematic since the proper function can only be determined when the short-circuit event is artificially induced. Since the short-circuit current or desaturation current is generally very much higher than the nominal current during normal operation, without other precautions, an artificially induced short-circuit would shorten the service life of the electrical components involved and possibly lead to an uncontrolled change in torque.